1. Field of the Invention
The invention relates to a method, a computer program and a rolling mill train for rolling a metal strip. The rolling mill train comprises N active rolling stands arranged one after the other in the rolling direction.
2. Description of the Prior Art
In principle, such methods, computer programs and rolling stands are known in the prior art. Thus, from the International Publication WO 2009/049964 A1, a rolling mill train having at least two rolling stands is known, wherein the metal strip as it passes the rolling stand undergoes in each case a thickness reduction, since the roll gap of the rolling stand is set in each case to a predetermined initial pass thickness. The strip tension, in particular between two rolling stands, is monitored and if necessary it is set appropriately by means of appropriate setting means. Before the entry of the rolling material head into the roll gap, the latter is set in the vertical direction substantially to the inlet-side rolling material head thickness. After the entry of the rolling material head into the roll gap, the latter is closed to a predetermined value, and substantially simultaneously with the closing, the peripheral speed of the working rollers is changed, in particular increased, depending on the size of the roll gap.
In reference to FIG. 3, the method shown therein, which is the prior art, is explained in greater detail below, without referring to a printed document. The starting point is a four-stand tandem rolling mill train 10, wherein an unwinder 8 is arranged upstream of said mill train and a winder 12 is arranged downstream of said mill train. The method shown in FIG. 3 for cold rolling a metal strip 200 provides that first all the stands of the tandem mill train 10 are moved out, so that first the metal strip with the strip head 210 is passed without thickness reduction through the roll gap of the rolling stand to the winder 12, where it starts to be wound. As the winding starts, a tensile stress is generated in the metal strip between the winder 12 and the unwinder 8; see FIG. 3c).
After the build-up of the tensile stress, the working rollers of the rolling stands are first all placed onto the metal strip 200, see FIG. 3d), before the rolling at the first stand starts, in which the working rollers of said stand are closed to a roll gap having a predetermined initial pass thickness; see FIG. 3e). The thickness jump in the metal strip caused in this manner by the first rolling stand then passes successively through all the subsequent rolling stands of the tandem train 10. In the process, successive starting of the rolling on the individual stands occurs, as soon as said thickness jump passes the respective stand; see FIGS. 3f and 3g). The last rolling stand of the tandem train is preferably set to the desired target thickness for the metal strip.
There are two essential reasons for carrying out this method: On the one hand, the force and work demand during rolling without tension is considerably higher than with tension and, on the other hand, especially in the case of the small thicknesses used in cold rolling, the strip very rapidly becomes uneven, if the roll gap profile does not fit the incoming profile of the metal strip, and the rolling material thus undergoes different elongations over the strip width. A metal strip with unevenness can as a rule not start to be wound or rolled further in a subsequent stand, that is it cannot undergo a further reduction in thickness.
The disadvantage of this method is that, at the strip head, a considerable length of the metal strip does not have the desired thickness, and therefore has to be scrapped as off-gauge length. A similar situation occurs at the strip end. Here the back tension is missing as soon as the strip leaves the unwinder 8 or as soon as the last windings of the coil are in contact. In the conventional mode of operation, the roll gap of the individual rolling stands is also opened here, and this also results in off-gauge lengths.